The invention relates generally to systems for providing a plurality of telephone circuits over a smaller number of metallic wiring pairs, and relates more particularly to a digital added main line system having improved ring generator, off-hook detector, and ring cadence replication capability for use in a system providing an added main line for a telephone subscriber over an existing conductor pair.
A typical telephone circuit is shown in FIG. 1. A central office provides two copper conductors shown as T and R (for "tip" and "ring") which extend in unbroken metallic continuity to the customer. Customer equipment is connected to the tip and ring conductors to complete the loop, often called a "local loop". The voltages, currents, and signaling methods employed in typical telephone local loops are well known and will be detailed herein only as required to explain the inventive added main line system.
Telephone customers, both residential and commercial, demand ever greater numbers of telephone circuits for voice communication, computer data exchange by modem, print information transmission by telecopier, credit card verification, and burglar alarm monitoring. FIG. 2 shows the historically most frequent outcome upon a customer's requesting the installation of an additional telephone line. In addition to the previously existing conductors T and R (here denoted T1 and R1) the telephone company installs (or assigns from previously installed spares) two additional conductors T2 and R2. These provide continuous metallic connection between central office equipment CO-2 and customer equipment 2.
As will be readily appreciated by those skilled in the art, the cost of the facilities (i.e. copper pair) associated with the added line increases with the distance d between the central office and the customer. Where d is large there is an incentive to consider other means of supplying the added line.
Feasibility of provision of the added copper pair T2, R2 is a function of the existing or expandable capacity of the conduit c. The conduits will typically have been designed and installed based on older, more moderate, projections of growth in demand. It is not uncommon in urban areas for every possible wire path in a conduit c to be filled, so that adding a copper pair requires construction of additional conduit at prodigious expense. This becomes a problem both for underground conduits and for wiring risers in buildings. Telephone companies are faced with the prospect of burying or stringing additional wire pairs from the central office to the customer premises, a time-consuming and expensive process which may only have to be repeated if the number of additional pairs installed is again outstripped by demand.
It frequently happens in present-day times that a telephone customer orders an additional telephone line and the telephone company is not able to install it since all existing copper wire pairs to the customer's premises are in use. Much attention has thus been paid in recent years to the problem of expanding the effective capacity of the installed infrastructure of copper pairs of telephone wire.
In the very long term trends in installation of fiber optic cable may ease some of these problems since the information bandwidth for a given cross-sectional area of fiber is many times that of copper. But near-term models for fiber penetration suggest that for many years and for many service areas only expansion of the capacity of the existing copper will permit meeting customer needs.
Attention has thus turned to the development of so-called "added main line" equipment. FIG. 3 shows one approach based on known analog circuitry. Tip and ring signals from two central office lines are provided to analog added main line equipment in the central office; functionally that equipment receives four conductors from the central office and provides two conductors to the customer. Other analog added main line equipment at the customer's premises receive the two conductors from the central office and provide four (or more) conductors for connection to customer equipment. Known analog added main line equipment may have any of several drawbacks. For example, the added main line equipment at the customer's premises typically requires provision of external power, as shown in FIG. 3. Thus the usual reliability of the telephone service even in the face of loss of local power is lost. Another drawback may be that one or both of the customer equipment connections is nonstandard. In one commonly used analog added main line system, for example, the second line cannot accommodate conventional telephone instruments and answering machines; the ringing signal is provided by a separate wire to the customer equipment which must be modified to accept the separate ringing signal.
In any added main line system, whether it is of the design shown in FIG. 3 or is of novel design, there must be so-called "ring generator" circuitry to generate the ringing signals that annunciate an incoming call to the customer equipment. (The terminology of "tip" and "ring" identifying the two wires of a pair has its origins in the contacts of a plug used to connect the loop; the term "ring" in this context has nothing to do with the voltages for ringing telephones.) There is thus a need for a ring generator which satisfies several constraints. The ring generator should be line-powered rather than requiring external power. The limited amount of power which can be passed over a conventional copper pair dictates that the ring generator's use of power extracted from the line be extremely efficient. Indeed the ringing portion of a telephone call is associated with the highest currents and voltages of any portion of the call.
In any added main line system, whether it is of the design shown in FIG. 3 or is of novel design, there must be so-called "off-hook detection" circuitry in the customer-side equipment. For example, when one of the customer lines needs to ring, the customer-side equipment generates the abovementioned ring signals. If the customer equipment takes the ringing line off-hook (e.g. picks up the telephone) it is necessary that the customer-side equipment detect this event. Known off-hook detection circuits typically have a large potential across the customer equipment, and have a large resistor in series with the line. And in many central office designs the ringing voltage is applied across the series combination of the large resistor and the customer equipment. The requirement of this large potential, together with the large resistor, limits the power efficiency of the system. As mentioned above, the customer-side equipment is desirably powered by energy transmitted over the copper loop, and so the power available for supervision of the off-hook status of the subscriber equipment is limited. There is thus a need for an off-hook detection system that works without the need of such a high potential. And there is thus a need for ringing arrangements that minimize the dissipation of ringing energy at locations other than the customer telephone equipment.
The tip and ring signals provided for the customer's equipment for each line should be such that conventional telephone instruments and accessories may be used without modification. It is also desirable that sufficient ringing current be provided that the customer may plug in as many devices that consume ringing current as would be possible with connections such as those of FIGS. 1 and 2. It is also necessary that the added main line system be able to detect the condition of customer equipment going off-hook (e.g. a telephone handset being picked up) despite the limited power available for testing for the off-hook condition.
The ringing signal provided to customer equipment in the United States is most commonly provided with a duty cycle or cadence of two seconds on, four seconds off. It has become commonplace in recent years, however, for telephone companies to generate other cadences, such as to provide distinctive ringing signals to indicate which of several telephone numbers had been used to reach the customer. It is thus desirable that the added main line apparatus be able to replicate, at the customer line, the ringing cadence provided at the central office.
There is particularly a need that all these conditions be satisfied reliably and without undue expense.